High performance cochlea implants have been developed, an example being shown in FIG. 1. Sound waves are received by microphone 21, sent along cables 22 to the speech processor 13, transformed into a coded signal that contains a very rapid pattern of small pulses, returned along the same cable to the transmitter 15, sent via radio transmission through the intact skin to the receiver 16, decoded and sent as a pattern of very rapid small electrical pulses to the electrodes 17 in the cochlea and delivered by the electrodes to stimulate the auditory nerve 18. Different parts of the nerve are stimulated according to the pitch of the sound waves received by the microphone. In response, the auditory nerve carries out its natural function and conducts nerve impulses to the brain. The brain receives the nerve impulses and interprets them as sound.
The step that involves delivery of electrical pulses to the auditory nerve is most effectively executed when the electrode contact is as close as possible to the auditory nerves that are particularly responsive to selected pitches of sound waves. Electrode carriers have been designed that are capable of being inserted along much of the length of the cochlea. For the Combo 40+ (Med-El), the standard distance over which the electrode pairs are arranged is 26 mm. In some patients, ossification of the cochlea has occurred such that it is not possible to insert an electrode carrier throughout the entire length of the cochlea. Therefore in these patients, the electrode carrier is shortened and the contacts are arranged over a shorter distance. (Med-El product information)
The most commonly used technique for inserting cochlear implants is to drill a bony channel corresponding to the scala tympani of the lower basal coil in a procedure referred to as a posterior tympanotomy. In ossified cochlea, the basal coil is obstructed so that it is not possible to reach all the target ganglion cells in the modiolus. Consequently the range of speech frequencies that may be registered by the patient with an ossified cochlea is reduced.
There is a need therefore for new approaches to accessing an increased number of ganglion cells in ossified cochlea.